Bottom Line:
The generation and transport of photocurrent in multilayer MoS2 are found to differ from those in other low-dimensional materials that only contribute with either photovoltaic effect (PVE) or photothermoelectric effect (PTE).In multilayer MoS2, the PVE at the MoS2-metal interface dominates in the accumulation regime whereas the hot-carrier-assisted PTE prevails in the depletion regime.Besides, the anomalously large Seebeck coefficient observed in multilayer MoS2, which has also been reported by others, is caused by hot photo-excited carriers that are not in thermal equilibrium with the MoS2 lattice.

ABSTRACTAs a finite-energy-bandgap alternative to graphene, semiconducting molybdenum disulfide (MoS2) has recently attracted extensive interest for energy and sensor applications. In particular for broad-spectral photodetectors, multilayer MoS2 is more appealing than its monolayer counterpart. However, little is understood regarding the physics underlying the photoresponse of multilayer MoS2. Here, we employ scanning photocurrent microscopy to identify the nature of photocurrent generated in multilayer MoS2 transistors. The generation and transport of photocurrent in multilayer MoS2 are found to differ from those in other low-dimensional materials that only contribute with either photovoltaic effect (PVE) or photothermoelectric effect (PTE). In multilayer MoS2, the PVE at the MoS2-metal interface dominates in the accumulation regime whereas the hot-carrier-assisted PTE prevails in the depletion regime. Besides, the anomalously large Seebeck coefficient observed in multilayer MoS2, which has also been reported by others, is caused by hot photo-excited carriers that are not in thermal equilibrium with the MoS2 lattice.

f4: Gate-dependent photocurrent in MoS2 transistor.(a) Photocurrent (IPC) line scans across the contact edge as indicated by the black dashed line in Fig. 3a at Vg from 0 V to 20 V and (b) Vg from 0 V to −20 V. The red arrows in panels (a) and (b) indicate the position of peak photocurrent. (c) Photocurrent peak position and amplitude in panels (a) and (b) varied with Vg. (d) The laser-power (P) dependency of the photocurrent with laser illuminated at the contact edge for Vg = 15 V and −15 V. The black and red lines are power-law fits with IPC ∝ Pα.

Mentions:
In order to gain more insights into the photocurrent generation mechanisms, we performed line-scan across the contact edge at a step size of 200 nm as shown by the black dashed line in Fig. 3a at various Vg (Figs. 4a–b). The peak position and peak amplitude of the photocurrents are extracted and plotted in Fig. 4c. The most striking feature in the plot is the movement of the peak position at the contact edge as Vg is decreased below −5 V. Concurrently, the peak amplitude decreases exponentially with Vg without sign flip. According to Fig. 3d, the thermoelectric photocurrent is expected to be negligible in the depletion regime (−15 V < Vg < −10 V) since S approaches zero in this region. As a result, PVE should dominate the photocurrent there. When Vg is changed from positive to negative, the built-in electric field reverses and the peak electric field at negative Vg moves away from the contact edge and extends more into the channel region compared to that at positive Vg (Supplementary Fig. S6). At Vg = −15 V, the photocurrent peak moves ~0.6 μm away from the contacts, which is in qualitative agreement with the movement of the simulated peak electric field. If the photocurrent generation is dominated by PVE in the depletion regime, the sign of photocurrent should reverse when (Vg − Vt) changes from positive to negative, which is however not observed experimentally. The contradicting polarity of the photocurrent suggests that the effect of the built-in field is less likely to be the dominant mechanism to determine the direction of the photocurrent in the depletion regime.

f4: Gate-dependent photocurrent in MoS2 transistor.(a) Photocurrent (IPC) line scans across the contact edge as indicated by the black dashed line in Fig. 3a at Vg from 0 V to 20 V and (b) Vg from 0 V to −20 V. The red arrows in panels (a) and (b) indicate the position of peak photocurrent. (c) Photocurrent peak position and amplitude in panels (a) and (b) varied with Vg. (d) The laser-power (P) dependency of the photocurrent with laser illuminated at the contact edge for Vg = 15 V and −15 V. The black and red lines are power-law fits with IPC ∝ Pα.

Mentions:
In order to gain more insights into the photocurrent generation mechanisms, we performed line-scan across the contact edge at a step size of 200 nm as shown by the black dashed line in Fig. 3a at various Vg (Figs. 4a–b). The peak position and peak amplitude of the photocurrents are extracted and plotted in Fig. 4c. The most striking feature in the plot is the movement of the peak position at the contact edge as Vg is decreased below −5 V. Concurrently, the peak amplitude decreases exponentially with Vg without sign flip. According to Fig. 3d, the thermoelectric photocurrent is expected to be negligible in the depletion regime (−15 V < Vg < −10 V) since S approaches zero in this region. As a result, PVE should dominate the photocurrent there. When Vg is changed from positive to negative, the built-in electric field reverses and the peak electric field at negative Vg moves away from the contact edge and extends more into the channel region compared to that at positive Vg (Supplementary Fig. S6). At Vg = −15 V, the photocurrent peak moves ~0.6 μm away from the contacts, which is in qualitative agreement with the movement of the simulated peak electric field. If the photocurrent generation is dominated by PVE in the depletion regime, the sign of photocurrent should reverse when (Vg − Vt) changes from positive to negative, which is however not observed experimentally. The contradicting polarity of the photocurrent suggests that the effect of the built-in field is less likely to be the dominant mechanism to determine the direction of the photocurrent in the depletion regime.

Bottom Line:
The generation and transport of photocurrent in multilayer MoS2 are found to differ from those in other low-dimensional materials that only contribute with either photovoltaic effect (PVE) or photothermoelectric effect (PTE).In multilayer MoS2, the PVE at the MoS2-metal interface dominates in the accumulation regime whereas the hot-carrier-assisted PTE prevails in the depletion regime.Besides, the anomalously large Seebeck coefficient observed in multilayer MoS2, which has also been reported by others, is caused by hot photo-excited carriers that are not in thermal equilibrium with the MoS2 lattice.

ABSTRACTAs a finite-energy-bandgap alternative to graphene, semiconducting molybdenum disulfide (MoS2) has recently attracted extensive interest for energy and sensor applications. In particular for broad-spectral photodetectors, multilayer MoS2 is more appealing than its monolayer counterpart. However, little is understood regarding the physics underlying the photoresponse of multilayer MoS2. Here, we employ scanning photocurrent microscopy to identify the nature of photocurrent generated in multilayer MoS2 transistors. The generation and transport of photocurrent in multilayer MoS2 are found to differ from those in other low-dimensional materials that only contribute with either photovoltaic effect (PVE) or photothermoelectric effect (PTE). In multilayer MoS2, the PVE at the MoS2-metal interface dominates in the accumulation regime whereas the hot-carrier-assisted PTE prevails in the depletion regime. Besides, the anomalously large Seebeck coefficient observed in multilayer MoS2, which has also been reported by others, is caused by hot photo-excited carriers that are not in thermal equilibrium with the MoS2 lattice.